1. An enclosure in which energy in a nonthermal form is converted to heat, especially such an enclosure in which heat is generated by the combustion of a suitable fuel.
2. An intensely hot place: the furnace of the sun; an attic room that is a furnace in the summer.
3. A severe test or trial: endured the furnace of his friends' blame after the accident.
[Middle English fourneyse, from Old French fornaise, variant of earlier fornais, from Latin fornāx, fornāc-, oven; see gwher- in the Appendix of Indo-European roots.]
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